Cicada invasion feeds forests

A huge troop of cicadas known as Brood X emerges in the eastern United States every 17 years, covering streets, cars and buildings with a crunchy coating of insects. Although they aren't welcome in the cities, the nutrients from the Magicicada carcasses provide a valuable boost to forest ecosystems, says a California ecologist.

When the insects appear, the adults burrow up from the soil to mate, then lay their eggs inside trees, allowing newborn larvae to travel back underground. Once the mating season ends, these adults die and their bodies litter the forest floor.

The most recent invasion took place last spring, in an area covering eastern states such as Tennessee and Kentucky. Experts estimate that, in affected areas, around 100 insects emerge from each square metre of ground, although this number can vary from fewer than 10 to more than 300.

Scientists had already noticed that in the years following the cyclical emergence of the cicadas, forest plants seem to have unusually high amounts of nitrogen in their leaves. Nitrogen is normally a limiting factor in the plants' growth.

So Louie Yang at the University of California, Davis, decided to investigate further. He measured the levels of nutrients in experimental soil plots to which he had added cicadas, and compared them with control plots that lacked the extra insects.

Bountiful bugs

To Yang, the idea of going into a forest to dispense dead cicadas by hand was anything but tedious. "Putting dead bugs on the ground does not sounds like fun for most people, but for me it's really exciting. It will produce results," he says.

Yang found that, a month after adding cicada carcasses, the numbers of bacteria and fungi known to feed on biological material in the soil had been boosted, and the amount of ammonium and nitrate available for plants to use had increased by up to three times.

He added 140 cicadas per square metre to a plot that contained a forest plant called the American bellflower (Campanulastrum americanum); the plants later had 12% more nitrogen in their leaves, compared with plots without cicadas, and produced seeds that were 9% larger.

The results directly prove the link between the cicadas and increased levels of nitrogen in plants, says Yang. The insects are generally known for their negative effects on forest plants: they eat large amounts of plant matter and lay their eggs within the trees. But Yang believes the swarms could have a beneficial effect on the ecosystem overall, by kick-starting forest growth every 17 years.

Similar 'resource pulses' are seen in other ecosystems, he points out, including dramatic plant growth after El Niño rainfalls, and the nutrient boost to riverbank communities when large amounts of salmon die after spawning.